How to stop webpack-isomorphic-tools after started?

Once I've started my server using

let request;

new WebpackIsomorphicTools(
    require('../../../webpack/webpack.config.isomorphic')
).server(basePath, () => {
    request = agent(server.listen());
});

How do I stop it? We use the above to start the server in our test environment to run tests against. I'm working on upgrading to a new version of Mocha which doesn't exit by default anymore once all tests are complete (it waits for you to cleanup as well), so I want to make sure I shut down the server after we don't need it anymore.